Many industrial articles are comprised of multiple components to economically improve their properties. Multi-component articles made from thermoplastic and thermoset materials are generally manufactured with a final melt-mixing extruder that homogenously combines the various components into an article such as a sheet, film, fiber, a bottle or an injection molded part, frequently called a preform. The article, particularly the preform, is often further processed to make another article such as a bottle, tray, jar, or bag.
As packaging demands become more complex, multiple components are needed to increase the functional properties of the package. Barrier to vapor or specific compounds such as oxygen is one of the more important of these properties. Oxygen barrier materials are expensive and it is therefore desirable to minimize their cost in the final package.
Reduced rates of oxygen transmission can be achieved using passive or active barrier techniques. Passive barrier techniques reduce the transmission rate of the vapor or liquid into the package. By contrast, active barrier techniques incorporate material(s) into the wall of the package that react(s) with the vapor or liquid of concern and thus prevents their passage through the container wall.
Current packages integrate the passive barrier material into a separate layer in the wall of the container. This is accomplished by using one extruder to melt a major component and form the article while a second extruder melts the barrier material and injects the barrier material in a separate layer of the article that forms the wall of the container. One can, for example, improve passive barrier properties by incorporating a polyamide layer and a polyester layer to make a multi-layer container or one can homogeneously blend with the polyester with the polyamide in the container wall as opposed to the polyamide being placed in a separate layer.
The active barrier technique involves the reaction of a component in the wall of a container with oxygen. Such a reaction has come to be known as oxygen scavenging. U.S. Pat. Nos. 5,021,515, 5,049,624, and 5,639,815 disclose packaging materials and processes utilizing polymer compositions capable of scavenging oxygen; such compositions include an oxidizable organic component, such as a polyamide like m-xylylene adipamide (commonly referred to as MXD6) and a metal oxidation promoter (such as a cobalt compound).
U.S. Pat. No. 5,529,833 describes another organic composition comprising an ethylenically unsaturated hydrocarbon oxygen scavenger catalyzed by a promoter such as a transition metal catalyst and a chloride, acetate, stearate, palmitate, 2-ethylhexanoate, neodecanoate or naphthenate counterion. Typical metal salts are selected from cobalt (II) 2-ethylhexanoate and cobalt (II) neodecanoate.
U.S. Pat. Nos. 6,406,766, 6,558,762, 6,346,308, 6,365,247, and 6,083,585 teach to functionalize the oxidizable component such as a polybutadiene oligomer and react it into the backbone of the major polymer matrix, such as polyethylene terephthalate and/or its copolymers, (commonly referred to as PET). Such a composition may be incorporated into the wall of the container as a separate layer of the container wall or comprise the entire wall.
Elemental or reduced metal scavengers are another active barrier technique. These metals, usually in the presence of a promoter such as sodium chloride, are not reactive with oxygen until exposed to moisture that triggers the reaction. The advantage of the metal scavenger is that a pellet containing a metal based scavenger will not react with oxygen unless placed in contact with moisture, a component that is external to the pellet. The use of an agent external to the pellet composition to initiate the reaction makes this a triggerable system. This is in contrast to the previously discussed organic systems which are active when the ingredients are combined to make the container or pellet. It is noted that there are some oxygen reactive compounds that have both an inherent reactivity with oxygen and also have a promotable and/or a triggerable reactivity as well.
Solid phase polymerization efficiently increases the molecular weight, as measured by intrinsic viscosity of both polyesters and polyamides. In the solid phase polymerization process the pellets are exposed to temperatures less than the temperature at which the pellets become liquid. This temperature exposure occurs in the presence of a driving force such as an inert gas or vacuum. The by-products of the polymerization reaction are removed from the pellet thus forcing an equilibrium increase in the molecular weight of the polymer. Because the polyester and polyamide are both pelletized during manufacture, U.S. Pat. No. 5,340,884 advocates combining them at the point where one or the other pellet is cut during its original manufacture. Combining the polyester and polyamide where one or the other is cut into pellets eliminates a subsequent extrusion and cutting step. However, combining the polyester and polyamide at the first cutting step requires that the subsequent polyester-polyamide multi-component pellet be subjected to and survive the thermal processing steps of crystallization, solid phase polymerization and drying. These thermal processing steps can occur from 40° C. to a temperature slightly less than the temperature at which the pellet becomes liquid, such as 1° C., or for more typically for commercial reasons, 5° C. below the temperature at which the pellet becomes liquid.
While U.S. Pat. No. 5,340,884 advocates combining the polyester and polyamide at the first cutting step, it notes that homogenously dispersed combinations of the polyamide and polyester in masterbatches, concentrates and pre-compounds cannot be exposed to solid phase polymerization conditions without destroying the molecular weight of the polyamide and bringing on dramatic color shifts. U.S. Pat. No. 5,340,884 minimizes this by using a pre-compounded polyamide concentrate to be blended with the polyester. While a pre-compounded concentrate may reduce some of the effects of subsequent thermal processing, it is not very effective. The polyamide-polyester concentrate suffers the same problem as the separate polyester polyamide pellets dried together.
The problem is compounded in the case of the organic scavenging system where the oxidation promoter is added when the polymers are melt blended. The system is reactive to oxygen and any subsequent thermal treatment, and even storage, must be done in the absence of oxygen.
In a crystallization process, the material in the pellet moves from being substantially amorphous to crystalline. Polymer pellets are usually amorphous but will crystallize when subjected to heat and time. Crystallinity is often expressed as percent crystallinity and is often interpolated by dividing the difference between the density of the material and the amorphous density of the material by the difference between the 100% crystalline density and the amorphous density (0% crystallinity). For polyethylene terephthalate or the polyethylene terephthalate copolymer, the amorphous density is 1.335 gm/cm3, the crystalline density is 1.455 gm/cm3 and the expression therefore for a given sample is:(Ds−1.335)/(1.455−1.335)where Ds is density of the sample in gm/cm3.
Crystallinity levels can also be determined by differential scanning calorimetry (DSC or D.S.C.) which relates the amount of heat required to melt the material to the amount of heat required to melt a sample of 100% crystallinity.
Pre-compounding oxygen reactive components into a pellet suffers the same limitations as the polyester-polyamide blend. Pre-compounding of oxygen reactive pellets is particularly expensive because the pre-compounded pellets are reactive with oxygen and must be processed, stored and transported in the absence of oxygen. Oxygen reactive pellets are therefore packaged in the absence of oxygen under nitrogen into sealed foil bags.
Additionally, the pre-compounded oxygen reactive pellet does not work for post thermal treatments, such as drying, crystallizing or solid phase polymerizing. A pre-compound of a polyester and an oxygen reactive material must be dried prior to final melt mixing. Thus, the pellets must be able to withstand the drying process. Drying with hot air significantly depletes the capacity of the oxygen reactive material to consume oxygen.
U.S. Pat. No. 6,406,766 teaches that post polymerization processes such as drying, devolatilization, crystallization, and solid state polymerization diminish the oxygen scavenging capability and teaches that the loss of oxygen scavenging activity can be minimized by performing such operations in the strict absence of oxygen and limiting the copolymer's exposure to oxygen after such operations.
In spite of its limitations, the current industrial approach pre-compounds an oxygen sensitive material with a promoter/catalyst in a masterbatch containing the major component of the finished article to create an oxygen reactive material, ships the oxygen reactive material in foil bags and subsequently dries the masterbatch in the presence of nitrogen or vacuum just prior to addition into the final melt mixing extrusion process.
One alternative utilizes separate pellets: one pellet containing the oxygen sensitive component and the other pellet containing the major component and the promoter. Accurate metering of the components is difficult because of stratification caused by various specific gravity and electrostatic properties of the pellets and pre-compounding the oxygen sensitive component with the other components and adding the promoter/catalyst during the final melt mixing step may eliminate storage costs but reintroduces the metering difficulties and separate feed systems.
U.S. Pat. No. 5,627,218, and U.S. Pat. No. 5,747,548 describe a compartmentalized (zoned) pellet wherein interactive or interreactive components are kept in separate compartments or zones of the pellet until the final melt mixing step. Interactive/interreactive components are chemical reactants which are thermally activated to participate in a chemical reaction upon utilization of the pellet in a molding operation. The type of reaction classified as interactive or interreactive are those components that have reactions which must be carried to completion so that the products can be standardized and macromolecular growth limited during molding to prevent the product from becoming too stiff to mold. By contrast, the compartmentalized pellet composition of U.S. Pat. No. 5,627,218 utilizes the slowness of reactions to permit easy mold fill before the reaction converts easy flow materials into less easy flow materials. The reaction rates of the interreactive/interactive compounds are almost universally slow. Upon melt mixing, the interreactions of the contained materials begin. The reaction times of the pellet contained reactants is generally long in comparison to the time required for the molding or extrusion process. The resulting increase of molecular size and linkage complexity enhances physical properties and largely takes place after the material is formed into final shape. Post molding cure can be allowed to take place slowly or an oven cure can be affected at any time.
Reactive extrusion processing is typical of thermosets. While in theory some polyester-polyamides may slightly react in what is known as trans-amination, the reaction would be very rapid and would certainly not build molecular weight or increase viscosity, nor would it continue after the melt-extrusion step. It is known that polyethylene terephthalate does not react with poly m-xylylene adipamide, MXD6.
U.S. Pat. Nos. 5,627,218 and 5,747,548 teach that the compartmentalized pellets contain one or more chemical reactants which are thermally activated to participate in a chemical reaction upon utilization of the pellets in a molding operation. Catalysts which enhance the reaction may also be included. In addition to at least two chemical reactants, the patents also contemplate non-chemically reactive additives such as reinforcing fibers, colors, lubricants and the like.
By keeping the interactive/interreactive components separate until melt mixing, a single pellet is used and the complicated feeding systems associated with the final melt-mixing step are avoided. Neither U.S. Pat. No. 5,627,218 nor U.S. Pat. No. 5,747,548 contemplates or discloses the use of a compartmentalized pellet when the reaction is with a compound external to the pellet, such as oxygen.
U.S. Pat. No. 6,669,986 discloses the use of the compartmentalized pellet to aid in the drying of non-crystallizable polyesters by surrounding them with a crystallizable polyester to prevent blocking or sticking. To accomplish this, U.S. Pat. No. 6,669,986 teaches that the components must be chemically similar to have utility upon subsequent melt processing. U.S. Pat. No. 6,669,986 neither discloses nor contemplates using the compartmentalized pellet to protect the products from the reaction with an external compound such as the oxygen in air.
U.S. Pat. No. 6,669,986 also discloses protecting the ingredients of the pellet from water when the water triggers the reaction of a metal based oxygen scavenger. As discussed earlier, this is protection from a triggering mechanism and does not disclose protection from reacting with a compound or when the promoter is within the pellet. However, U.S. Pat. No. 6,669,986 does not teach protection of the components from reacting with oxygen.
U.S. Pat. Nos. 5,747,548 and 6,669,986 both describe the types of compartmentalized or zoned pellets and how to make such compartmentalized or zoned pellets. Examples of compositions not contemplated by the prior art are those pellet compositions when at least one of the components reacts with compounds in the environment such as the oxygen found in air.